A measurement device may count individual photons incident on the measurement device to measure an intensity of a light (e.g., an image sensor). For example, an image sensor may count incident photons until a threshold count of incident photons is reached. A measure of light intensity and/or flux may be obtained through counting the number of incident photons. The measurement device may determine the light intensity through a digital counter. A threshold detector may detect a count threshold from the digital counter in a time domain imaging operation.
A circuit to implement the digital counter and the threshold detector may be large in layout area. For example, the circuit may include 200 to 300 transistors for a 10 bit digital counter and threshold detector. This may increase the layout area when implemented inside a pixel of the image sensor. The large layout area of the circuit may prevent an implementation of the circuit inside a pixel of the measurement device. An inability to use the circuit inside the pixel may limit a speed of operation of the measurement device (e.g., frame rate). If the circuit is used inside the pixel, the circuit may increase the pixel size and/or may reduce a resolution of an output of the measurement device (e.g., image resolution). An increased pixel size may also reduce the number of pixels in an array of pixels forming the image sensor for a fixed array area.
Incrementing an upper count limit of the circuit may require an increase in the number of amplification and switching devices (e.g., transistors). For example, increasing the count by one bit may require an addition of a flip flop. The increase in the number of amplification and switching devices associated with an increment in upper count limit of the circuit may limit the dynamic range of the measurement device. Consequently, the measurement device may not operate efficiently in high performance applications.
In satellite imaging applications, an image sensor device may utilize time-delay integration (TDI) to increase sensitivity due to low light conditions or due to objects moving at a high speed. As such, the image sensor device may require a short integration time to avoid blurring of the image. Conventional TDI systems may use charge-coupled devices to facilitate charge transfer, which is required in TDI to shift a charge from one pixel to another in a manner synchronized with the movement of an object being captured by the image sensor device.
However, conventional CCD readout techniques are very slow, and have many disadvantages when compared to deep-submicron CMOS image sensors. Also, due to the low sensitivity of conventional CCD pixels, a large number of time-delay integration shifts may be required to increase the effective integration time.